1. Field of the Invention
The invention pertains to a method for polarizing materials to produce electrets for devices.
2. Art Background
Electrets are used in a wide variety of devices such as transducer devices, transformers, electric motors, and xerographic copying machines. (Electrets, for purposes of this disclosure, are electrically polarized bodies whose polarization persists after being produced.) The electret-containing transducer devices include, for example, microphones, loudspeakers, pressure sensors, touch sensitive keyboards and heat sensors. Furthermore, transducer devices such as electrostatic microphones or loudspeakers often include a thin film electret as the vibrating element of the device.
Materials employed in electrets are not inherently polarized but are polarizable. There are a wide variety of techniques for producing the desired polarization. For example, charges are injected into charge traps within or on the surface of the polarizable material, free charges are separated within the material, or dipoles of the material are aligned. Materials suitable for electrets and polarizable by charge injection techniques include polyesters such as polyethylene terephthalate and fluorocarbons such as fluorinated polyolefins. Polyvinylidene fluoride (PVDF) is exemplary of electret materials which are polarized by aligning dipoles of the material. Polarized PVDF is a commonly employed electret material because it exhibits both piezoelectric (an electrical signal is produced in response to an applied stress) and pyroelectric (an electrical signal is produced in response to heat) properties.
One technique for polarizing a body, e.g., a sheet, of polarizable material for electret fabrication involves placing the body between two conducting, e.g., metal, electrodes, and applying a voltage across the electrodes. If the body includes free charges or unoriented dipoles, then the electric field existing between the electrodes induces the charge separation or dipole alignment necessary to achieve polarization. If the body includes charge traps, then placing the body in contact with one of the metal electrodes results in a flow of charges from the electrode into the charge traps, also producing polarization.
One problem often encountered with the previously described polarization technique is large-scale dielectric breakdown. This typically results in the destruction of a significant portion of the polarizable body (a hole several millimeters in diameter is burned through the body) and results in either no polarization or an undesirably low degree of polarization in the undamaged portions of the body. It is believed that breakdown is generally initiated in a region of the body exhibiting a defect leading to reduced electrical resistance, e.g., a pinhole, decreased thickness, or low dielectric strength. A sufficiently large, applied polarizing voltage produces a short circuit (of charges distributed on the surfaces of the electrodes facing the sheet) through the defective region, reducing the polarizing voltage across the nondefective portions of the sheet, and thus precluding effective polarization. The short circuit also involves an avalanche-like effect which first produces a hole typically several micrometers in diameter through the body, and ultimately leads to catastrophic failure.
A method for mitigating the effects of breakdown as described by T. T. Wang and J. E. West in "Polarization of poly(vinylidene fluoride) by application of breakdown fields", Journal of Applied Physics, 53 (10), 1982, involves inserting a layer of dielectric material, e.g., a layer of soda-lime glass, between the body of material to be polarized and one of the conducting electrodes. (The dielectric insert either directly contacts one surface of the sheet, or a metallized surface of the sheet.) Breakdown, if it occurs, is thus limited to a relatively small region, typically no more than a few micrometers in diameter, and polarization occurs in the remaining portions of the sheet.
It is established doctrine that the dielectric insert functions as distributed resistive elements, and that only such distributed resistance prevents large-scale breadown. That is, each volume of the dielectric insert resistively limits current flow through an adjacent coextensive volume of the polarizable material. Thus, the dielectric insert allows a higher polarizing voltage across each volume without breakdown and results in a concomitant enhanced degree of polarization. Additionally, the dielectric insert resistively inhibits lateral current flow along the insert-electrode interface (provided the sheet is unmetallized, or is metallized and has suffered a local breakdown producing a discontinuity in the metal adjacent the breakdown region). Thus, in the event of a local breakdown, adverse consequences are limited by preventing charges distributed on the electrode surface from flowing along the insert-electrode interface to the local breakdown region.
While the use of a dielectric insert has many advantages, improvements are possible. For example, a greater degree of polarization uniformity, and shorter polarization times (the time required to achieve the desired degree of polarization) are desirable.